1. Field of the Invention
The present invention relates to an image forming apparatus mounting a fixing device used for an electrophotographic image forming process, as well as to an image forming method.
2. Description of the Related Art
In a general electrophotographic image forming process, a toner image formed on a medium (e.g. sheet) is heated and pressed, so that the image is fixed. A fixing device for fixing the toner image in this manner typically has a heating roller as a heating rotating body and a pressure roller as a pressurizing rotating body. The heating roller is heated by a heat source such as a heater.
In such a fixing device, in order to have a not-yet-fixed toner image fixed on the medium, surface temperature of the heating roller must be maintained at a prescribed value. The surface temperature of the heating roller, however, decreases because of heat transfer, for example, from the surface of heating roller to the medium. Therefore, the surface temperature of heating roller is detected using a temperature sensor such as a thermistor, and based on the result of detection, the heat source is controlled such that the surface temperature of heating roller is maintained at the prescribed value.
Further, the pressure roller that rotates in coordination with the heating roller also has its surface temperature decreased, as it is deprived of heat as sheets as media are fed. If images are formed continuously on a plurality of media (when sheets are fed continuously), the temperature of pressure roller decreases significantly, resulting in undesirably tight curl of the medium or a low-temperature offset. Such influence is more evident in a low temperature environment.
As a countermeasure to such a curl or low-temperature offset derived from decrease in surface temperature of pressure roller, sometimes an arrangement is used in which the surface temperature of pressure roller is detected by using a temperature sensor such as a thermistor and based on the result of detection, the pressure roller is heated using a heat source such as a heater. Alternatively, methods such as making longer the warm-up period immediately after activation, increasing a set value or values related to temperature control, and reducing yield immediately after activation are sometimes used to reduce decrease of the surface temperature of pressure roller and thereby to maintain good fixing characteristic.
Specifically, by making longer the warm-up period immediately after activation, heat can be accumulated in the pressure roller, so that drop of surface temperature of the pressure roller can be reduced. By increasing the set value or values related to temperature control, fixing characteristic can be improved. Further, by reducing yield immediately after activation, time interval of feeding media becomes longer, so that the pressure roller can be heated while the medium is not fed and, hence, the surface temperature drop of pressure roller can be reduced.
The following prior art references related to such temperature control of fixing devices have been known.
Japanese Laid-Open Patent Publication No. 2009-276549 discloses an arrangement having an auxiliary heater which is electrically conducted only at the time of warm-up until the fixing roller in a low-temperature state reaches a prescribed fixing temperature.
Japanese Laid-Open Patent Publication No. 2009-058773 discloses an arrangement in which paper feed interval is made wider immediately after activation if degree of humidity is high.
Japanese Laid-Open Patent Publication No. 2005-099373 discloses an arrangement in which, when the degree of humidity is low, warm-up is continued until that one of fixing members which has higher rate of temperature increase reaches a prescribed temperature, and when the degree of humidity is high, additional warm-up for a prescribed time period takes place after the fixing member which has higher rate of temperature increase reached a prescribed temperature. The additional warm-up is continued until timer-count ends, or until that one of the fixing members which has lower rate of temperature increase reaches a prescribed temperature.
Japanese Laid-Open Patent Publication No. 2009-265154 discloses an arrangement in which, at the time of warm-up after power on or at the time of returning from sleep mode for saving power consumption, if printing of sheets is received with the operation mode set to a normal mode, speed of printing at the fixing roller is set lower than the normally set speed generally set in the normal mode and the set temperature for fixing for the fixing roller is set lower than the normally set temperature generally set in the normal mode, printing operation is started and, when the fixing temperature reaches the normally set temperature, the speed of printing at the fixing roller is set to the normally set speed.
Japanese Laid-Open Patent Publication No. 2009-180755 discloses an arrangement in which speed of rotation of the fixing roller in the warm-up period can be set to two or more speeds of rotation and, based on a degree of humidity detected by a humidity sensor, the speed of rotation of the fixing roller in the warm-up period is set to one of the two or more speeds of rotation, such that the speed of rotation becomes higher if the detected degree of humidity is higher.
Japanese Laid-Open Patent Publication No. 2009-037077 discloses an arrangement having humidity detecting means for detecting environmental humidity of ambient surrounding, in which speed of rotation of the pressure roller during warm-up operation is changed based on the degree of humidity detected by the humidity detecting means.
Japanese Laid-Open Patent Publication No. 2008-102279 discloses control of rotation number at the time of warm-up based on temperature and humidity. More specifically, when the degree of humidity is low, rotation of the fixing roller is made slower, and if the temperature is high, rotation of the fixing roller is made slower.
Japanese Laid-Open Patent Publication No. 2007-183686 discloses a method of reducing temperature drop of the fixing roller by changing interval of paper feed.
The above-described methods of making longer the warm-up period immediately after activation, increasing set values related to temperature control and reducing yield immediately after activation have much influence on the user. Specifically, the method of making longer the warm-up period immediately after activation may require excessively long time to increase the temperature of pressure roller unless the pressure roller can be heated with high efficiency. The method of increasing set values related to temperature control increases thermal energy consumption. Further, the method of reducing yield immediately after activation leads to waste of time of the user.